1. Field of the Invention
The present invention relates to a process for mutual separation of platinum group metals (PGM), more particularly a process for mutual separation of PGM from a raw material which also contains impurity elements, wherein highly stable compounds and steps are used to efficiently remove the impurity elements while preventing increase of impurity content relative to that of the PGM in the mother liquor and also preventing decomposition of a chloro complex, and palladium, platinum, iridium, ruthenium and rhodium are separated in such a way that each of the separated PGM has a sufficient purity to be a commercial product.
2. Description of the Prior Art
PGM are scarce resources, and production of the natural minerals, e.g., platinum ores containing these metals at a high concentration, is limited. The raw materials for these metals produced on a commercial scale are mostly byproducts from refining of nonferrous metals, e.g., copper, nickel and cobalt, and various spent catalysts, e.g., those for treating automobile exhaust gases.
The byproducts from nonferrous metal refining contain PGM, e.g., platinum, palladium, iridium, rhodium, ruthenium and osmium present in the refining raw materials in trace quantities. They are concentrated, for their chemical properties, in the sulfide concentrates and the crude metals of the major metals, e.g., copper and nickel. They are separated in the form of precious metal concentrate containing these metals as the residue from a major metal recovery process, e.g., electrolysis.
The concentrate normally contains, in addition to copper and nickel as the major metals, other components, e.g., precious metals (e.g., gold and silver), 16 group elements (e.g., selenium and tellurium) and 15 group elements (e.g., arsenic), which are present at a higher concentration than PGM. Recovery of PGM follows recovery of gold and silver, which discharges a residue containing these metals together with impurity elements. The commercial process for separation/recovery of PGM from the above-described starting material containing them normally involves leaching in a solution and a subsequent separation process, e.g., solvent extraction or adsorption, by which they are separated mutually and refined.
Several processes by which PGM are separated mutually and refined have been proposed or implemented, and these processes and problems involved therein are described.
The processes for separation of individual PGM on a commercial scale from a raw material containing them may be represented by the one based on, e.g., the following steps (a) to (g) carried out in this order, with solvent extraction serving as the main separation technique:    (a) a raw material containing PGM is leached with aqua regia or chlorine to prepare an aqueous solution containing them,    (b) the resulting aqueous solution is heated in the presence of nitric acid or the like as an oxidant, to distill off osmium,    (c) the remaining solution is neutralized to a weakly acidic state, and heated in the presence of sodium hydrochlorate, chlorine or the like as an oxidant, to distill off ruthenium,    (d) the remaining solution is treated to have a hydrochloric acid concentration increased to around 3 mols/L, and brought into contact with diethylene glycol dibutyl ether to selectively extract gold,    (e) the resulting raffinate is brought into contact with a sparingly water-soluble alkyl sulfide to extract palladium,    (f) the resulting raffinate is treated to reduce the iridium (IV) ion to iridium (III) ion, and brought into contact with tributyl phosphate to extract platinum, and    (g) the resulting raffinate is treated to oxidize the iridium (III) ion to iridium (IV) ion, and brought into contact again with tributyl phosphate to extract iridium, leaving the rhodium in the raffinate.
These processes based on solvent extraction involve the following challenges to be overcome.
(1) Prevention of Increase of Impurity Content Relative to that of the PGM in the Mother Liquor
These processes are based on the common concept of selectively separating gold and PGM while leaving other impurity elements in the mother liquor. As a result, impurity content relative to that of the PGM increases gradually as the process proceeds. For example, impurity elements are present frequently at an as high as 10 to 100 times higher content than rhodium and iridium totaled in the raffinate discharged from the rhodium/iridium separation step as the final stage of the process, making refining of these elements substantially difficult. Therefore, recovery of a PGM having a sufficient purity to be a commercial product needs a sophisticated process, number of required refining steps increasing as residual impurity element content increases, increasing loss of the PGM separated out together with impurity elements and hence decreasing the final yield.
In solvent extraction with diethylene glycol dibutyl ether as an extractant, for example, hydrochloric acid is normally kept at 3 mols/L or less in the aqueous phase, at which gold is selectively extracted but other impurity elements are little extracted and remain in the raffinate. In other words, impurity elements other than gold are separated insufficiently from PGM.
(2) Use of Highly Stable Compounds and Steps
Ruthenium separation by distillation produces ruthenium oxide (VIII) gas, which is highly explosive and reactive with an organic compound. Therefore, it needs a system of highly corrosion-resistant material, e.g., quartz glass, which tends to push up the investment.
Refining of the stripping liquor containing iridium discharged from the final step is normally based on reduction separation with mercury (I) chloride as a reductant for its high efficiency of separating iridium from the other coexisting PGM. It is however an environmentally problematical step.
(3) Prevention of Decomposition of a Chloro Complex of a PGM in the Mother Liquor
Separation of ruthenium by distillation is frequently combined with solvent extraction. It is essential for this process to once neutralize the whole liquid, and, after the distillation step is completed, to increase concentration of free hydrochloric acid in the whole liquid. This should increase chemical consumption and, at the same time, capacity of the subsequent system because of the greatly increased liquid volume.
Each of the PGM is kept in the form of chloro complex, which is more suitable for solvent extraction than any other form and is resistant to hydrolysis resulting from changed pH level. However, the above process, involving neutralization and heating, tends to decompose the complex, and the decomposed product is difficult to be returned back to the original form even in the presence of newly added hydrochloric acid. This should decrease efficiency of separating the PGM by extraction.
For processes which use an adsorbent, on the other hand, various adsorbents and processes using them have been proposed. They may be represented by the following ones, each of which generally has one or more problems from a practical standpoint.    (1) Japanese Patent No. 3,291,203 (pages 1 and 2), for example, discloses a process which involves adsorption of a mixture of PGM in the form of aqueous solution on a chromatography medium, e.g., glycol methacrylate, and subsequent elution with an acidic solution for separating the individual elements. This process has practical problems resulting from very low adsorption capacity of the medium, which greatly increases the system capacity for recovering unit mass of each element. When 2 mL of a starting solution containing PGM at 0.3 g/L is to be treated, for example, a very large column, 10 mm in diameter and 300 mm in length, is required.    (2) JP-A-2001-98335 (pages 1 and 2), for example, discloses a process which involves adsorption of a mixture of PGM in the form of aqueous solution on an ethylene glycol/methacrylic acid copolymer, oligoethylene glycol or glycidyl methacrylate pentaerythritol-dimethacrylate copolymer, and subsequent elution with an elutant of hydrochloric acid containing an oxidant to separate rhodium and then with an elutant of hydrochloric acid containing a reductant to separate platinum and iridium. This process can achieve separation of iridium, whose adsorption characteristics notably change as it is oxidized or reduced, but has practical problems resulting from difficulty in separating other PGM mutually.    (3) JP-A-9-203792 (pages 1 and 2), for example, discloses a process which involves adsorption of a mixture of nitro complex anion of each PGM in the form of aqueous solution on an anion-exchange resin, and subsequent elution with thiourea, ammonia or the like to separate individual PGM stepwise. However, this process has practical problems resulting from difficulty in separating the PGM mutually to an extent that each of the separated elements has a sufficient purity to be a commercial product, because they are in the form of a nitro complex and similar to each other in chemical properties.    (4) JP-A-2001-516808 (pages 1 and 2), for example, discloses a process which involves extraction of a mixture of PGM in the form of aqueous solution with 4-methyl-2-pentanone to separate impurity elements, e.g., gold, tellurium and iron, passing the resulting solution over a medium mainly composed of a methacrylic acid ester gel, after it is adjusted at an oxidation-reduction potential of 500 mV or hydrochloric acid concentration of 5.5 to 6.5 mols/L, and subsequent elution with 6 mols/L hydrochloric acid to separate mixture of iridium, rhodium and ruthenium, palladium, platinum and osmium, in this order. This process, although individually separating palladium, platinum and osmium, has a problem of needing a separate step for separating iridium, rhodium and ruthenium from concomitantly separated copper, bismuth, lead and arsenic as major impurity elements, and another problem of greatly increased impurity element content relative to that of PGM after palladium and platinum as the major PGM are separated. Moreover, 4-methyl-2-pentanone as the extractant used in the first stage of this process is greatly lost in the process because of its very high solubility in water (19 g/L at 20° C.) and has safety-related problems resulting from its very low flash point of 17° C.
Under these circumstances, there have been demands for processes for mutual separation of PGM from a raw material which also contains impurity elements, wherein highly stable compounds (with respect to flash point and toxicity) and steps are used for solvent extraction of improved practicality to efficiently remove the impurity elements while preventing excessively increase of impurity content relative to that of the PGM in the mother liquor and also preventing decomposition of a chloro complex, and palladium, platinum, iridium, ruthenium and rhodium are separated mutually in such a way that each of the separated PGM has a sufficient purity to be a commercial product.